Vector halftone methods are typically used to render color separations for the printing of images in an inkjet printer. In these methods, a single stochastic screen is used to render the continuous tone data for multiple and potentially all of the color separations. As used in this document, “continuous tone data” refers to multi-bit digital data and is also denoted as contone data in the art. For example, an eight-bit byte can be used to identify an intensity level from zero to 255 for a primary color used in a printer to form images. For each output pixel, the input continuous tone color values for the primary colors corresponding to a same pixel are used to calculate the combinations of colors that can be formed with the continuous tone color values. These color combinations are added in some predetermined order and compared to a threshold in the stochastic screen that corresponds to the pixel after each combination is added. When the sum of the color combinations exceeds the corresponding threshold in the stochastic screen, a position in one or more binary maps for the last added color combination corresponding to the pixel is assigned a binary ‘1’. Thus, the rendering process converts multi-bit data values for the primary colors in the original digital image to binary maps for each primary color used to print the image. Where a binary one occurs in the map for a color, a drop of that ink is ejected to form a pixel and where a binary zero occurs in the map for the color, no drop of that ink is ejected. The binary maps for the colors are used to operate the inkjets in the printheads of the printer. The rendering of continuous tone data to produce binary maps for colors is known as “halftoning.”
Recently, printheads have been developed with inkjets that eject ink drops with variable volumes. For example, some printheads have inkjets that can eject ink drops with three different volumes. The inkjets are operated to eject differently sized drops by altering a parameter of the firing signal used to operate an inkjet. For example, firing signal waveforms having different amplitudes or frequencies can operate inkjets to eject ink drops of different sizes. In other known printers, the inkjets in the printheads can be operated to eject a variable number of drops having a constant volume in a single pixel location. Consequently, any position having a binary ‘1’ in a binary map produced by a vector halftone rendering process can be formed on an ink receiving surface with one or more ink drops in some printers or different pixels can be formed with differently sized drops. Modifying a vector halftone rendering process to optimize the variable printing parameters to form a pixel would be beneficial.